1. Field of the Invention
This invention relates to gassing compositions and a method for the preparation of gassed water-in-oil emulsion explosives compositions.
2. Background Information
Emulsion explosives compositions are well known in the explosives industry. The water-in-oil emulsion explosive compositions now in common use were first disclosed in U.S. Pat. No. 3,447,978 (Bluhm) and comprise as components:                (a) a discontinuous aqueous phase comprising discrete droplets of an aqueous solution of inorganic oxygen-releasing salts:        (b) a continuous water-immiscible organic phase throughout which the droplets are dispersed;        (c) an emulsifier which forms an emulsion of the droplets of oxidiser salt solution throughout the continuous organic phase; and optionally        (d) a discontinuous gaseous phase and/or closed cell void material.        
Emulsion explosives compositions are often blended with a solid particulate oxidiser salt such as ammonium nitrate (AN) prills or particles, which may be coated with or contain fuel oil (FO) to form a low cost explosive of excellent blasting performance. Such compositions are described in Australian Patent Application no. 29408/70 (Butterworth) and U.S. Pat. Nos. 3,161,551 (Egly et al.), 4,111,727 (Clay), 4,181,546 (Clay) and 4,357,184 (Binet et al.).
In water-in-oil emulsion explosives compositions, emulsifiers are used to decrease interfacial tension between the aqueous and oil phases. Molecules of the emulsifier locate at the interface between the aqueous droplet and continuous hydrocarbon phase. The emulsifier molecules are oriented with the hydrophilic headgroup in the aqueous droplet and the lipophilic tail in the continuous hydrocarbon phase. Emulsifiers stabilise the emulsion, inhibiting coalescence of the aqueous droplets and phase separation. The emulsifier also inhibits crystallisation of the oxidiser salt within the droplets which crystallisation can lead to emulsion breakdown and reduction in detonation sensitivity of the emulsion explosive composition.
A variety of emulsifier types and blends are known in the art. For example Australian Patent no. 40006/85 (Cooper & Baker) discloses water-in-oil emulsion explosive compositions which contain a conductivity modifier which may also act as an emulsifier. Included among such conductivity modifiers are condensation products of poly[alk(en)yl] succinic anhydride (PiBSA) with amines such as ethylene diamine, diethylene triamine and ethanolamine.
Such conductivity modifiers/emulsifiers enable the preparation of particularly stable emulsions which are suitable for blending with solid particulate oxidiser salts such as ammonium nitrate (AN) or ammonium nitrate and fuel oil blends (ANFO). The stability of emulsion explosives compositions prepared using such poly[alk(en)yl]succinic anhydride derivatives as conductivity modifiers/emulsifiers enables the preparation of unsensitised emulsion phase (EP) compositions at a dedicated plant under controlled conditions and transport of that EP to the mine site for sensitisation and use.
In general water-in-oil or melt-in-oil emulsion cannot be detonated unless they are sensitised. Sensitising may be carried out by mixing the emulsion with a high explosive such as trinitrotoluene or nitroglycerine or by incorporating small voids into the emulsion which act as hot spots in the detonation. The latter is the preferred method for sensitising a water-in-oil or melt-in-oil emulsion explosive composition.
The most common methods currently used to incorporate voids and sensitise a water-in-oil emulsion composition or emulsion/AN/ANFO blend include in situ gassing using chemical agents, the incorporation of closed cell void material such as microballoons or a mixture of both.
Suitable chemicals for the in situ generation of gas bubbles suitable for use in water-in-oil emulsion explosives include peroxides such as hydrogen peroxide, nitrite salts such as sodium nitrite, nitrosamines such as N,N′-dinitrosopentamethylenetetramine, alkali metal borohydrides such as sodium borohydride and bases such as carbonates including sodium carbonate.
Perhaps the most widely used chemicals for the in situ generation of gas bubbles are nitrous acid and its salts which react under conditions of acid pH to produce nitrogen gas bubbles. Accelerators such as thiocyanate salts, iodides, sulphamic acid or its salts or thiourea may be used to accelerate the reaction of a nitrite gassing agent. The accelerator may also be consumed in the reaction.
One of the problems with this commonly used gasser system of the prior art is that nitroso species generated during the gassing reaction may react with functional moieties on the headgroup of the emulsifier. Functional moieties on the emulsifier headgroup such as certain primary and secondary amines, amides, carboxylic acids, esters and anhydrides are particularly vulnerable to attack by nitroso species. Reaction by nitroso species with the moieties of the headgroup causes chemical changes in the emulsifier which may have a deleterious effect on the emulsifying capability of the emulsifier. As a result the interfacial tension of the droplets of the discontinuous aqueous phase may decrease, causing crystallisation of the of oxidiser salt within the aqueous phase droplets and degradation of the emulsion, possibly even to the point breakdown of the emulsion into separate aqueous and oil phases.
The problem of emulsifier reaction with gassing agents is referred to in Australian Patent Application no. AU-A-77589/94. AU-A-77589/94 relates to chemical gassing using sodium nitrite and teaches that “The commonly used chemical gassing reaction can thus not be used to gas the known PiBSA-based explosive emulsions.” In effect, this may lead to the need for different EP's to be used for gassed and ungassed products comprising emulsion explosives compositions and preclude the use of PiBSA derivative emulsifiers in nitrite gassed emulsion explosives compositions and hence the emulsion stability advantages provided by such emulsifiers.
Another problem associated with gassing agents, including nitrite gassing agents of the prior art is the difficulty of evenly distributing the gassing agent throughout the emulsion. International Patent Application WO-89/02881 attempts to address this problem by mixing into the main body of emulsion, a nitrite gassing agent which is also in the form of an emulsion.
One of the drawbacks of using an emulsion gassing agent is that the emulsion gassing agent dilutes both the aqueous and oil phases of the main body of emulsion, reducing the blasting power of the emulsion explosive formed.
One of the other problems associated with the gassing methods and gasser compositions of the prior art is that they had to be added in extremely high proportions to reduce emulsion density to very low densities such as, below 1 g/cc. The presence of extremely high proportion of gasser compositions often adversely affected emulsion stability due to dilution of the continuous or discontinuous phase of the emulsion. If either or both of these phases are overly diluted, there may not be sufficient emulsifier present to maintain an emulsion structure.